Review the reasons for setting `abi-version`

[philthompson10]

Normally a project will specify abi-version in its pyproject.toml file if it has a minimum ABI requirement. Otherwise SIP will use the .0 version of the largest major version it knows about. Both PyQt5 and PyQt6 explicitly specify a minimum version, other PyQt projects do not.

PyQt-builder will provide a default ABI version which is usually (but not always) the version support by the version of SIP that was current at the time. However this is confusing and somewhat arbitrary. It should only provide a default major version, ie. 12 or 13, or (preferably) not specify anything at all.

[philthompson10]

With the current version of SIP (v6.9.1) the versions should be v12.15 and v13.8 to get all current bug-fixes (ie. the same values used explicitly by PyQt5 and PyQt6 respectively).

Once the SIP issues 63, 64 and 65 are resolved, and PyQt5 and PyQt6 updated to use the new directive, then these can be removed completely.